1. Field of the Invention
This invention relates to construction of photovoltaic arrays and other apparatus requiring electrical connection between metal parts.
2. Prior Art
This invention has been devised for use in installing a photovoltaic array composed of a number of photovoltaic modules that are assembled onto a larger mounting structure. The frames of the individual photovoltaic modules and the structural pieces on which the modules mount are generally made out of aluminum. The aluminum is anodized to resist corrosion. As in any source of electrical power, to ensure safety the metal pieces must be bonded together. Bonded is used here in the technical sense to mean permanently joined to form an electrically conductive path that ensures electrical continuity and has the capacity to safely conduct any current likely to be imposed. Although the frames of the modules are directly bolted or clamped to the mounting pieces, the anodizing insulates the pieces so that they are not electrically connected together.
A common practice in the industry is to install a separate grounding lug on each piece that is anodized. The grounding lug is attached to the sheet metal frame of the modules with a thread forming stainless steel screw. Since the screw cuts into the aluminum it forms an air-tight connection which will maintain good electrical connection over time. A common sheet metal thickness is 0.080 inch and a common screw size is 10-32 so that the screw therefore only makes connection on about 2½ threads. This gives a marginally acceptable surface contact area in terms of mechanical strength and electrical conductivity. It would be desirable to have a design where the electrical contact area can be made much larger.
The stainless steel screw contacts a star washer which in turn contacts the grounding lug body. The grounding lug accepts a copper wire which is forced to contact the grounding lug by a stainless steel set screw. There are thus four connection points which must be made, sheet metal to screw, screw to star washer, star washer to lug, and lug to copper wire. It would be desirable to have a design where there are only two electrical connection points.
Copper wire is strung between grounding lugs on all the metal pieces and eventually to a ground electrode. The grounding lugs themselves are expensive and time consuming to install and the wiring adds both material and labor cost which increase the price of the overall system. It would be preferable if the electrical connection was made directly between the metal pieces when they are assembled together.
Star washers, such as that in FIG. 1, are generally available for making electrical connections. They are constructed so that they can be punched and formed out of flat metal stock. The forming process makes a number of internal or external teeth that are usually twisted so that they extend above and below the plane of the washer body. Star washers make electrical connection to an adjacent piece because they tend to dig in to the adjacent piece as they are rotated and compressed by a nut and bolt being tightened. In the intended application, they would be inserted in between the two pieces of metal to be bonded. They will therefore be separated from the nut and bolt by a layer of material and so will not rotate in the usual manner and therefore will be less effective. Star washers are intended for applications where all they have to break through is grease or dirt on a metal surface, but anodized aluminum is a very hard material. The teeth of a star washer will act like springs and deform back to a flat surface when compressed. This is desirable for most applications because the spring action maintains contact. In this application, however, depending on the thickness of the anodizing, the star washer may or may not penetrate the anodizing to make connection with the underlying aluminum metal. Some examples of this type construction are found in U.S. Pat. No. 5,453,027 (Buell et al., 1995),U.S. Pat. No. 5,620,290 (Homfeldt et al., 1997), and U.S. Pat. No. 6,939,097 (Carr et al., 2005), Shapes other than washers may also use these twisted type teeth, as in the clips of U.S. Pat. No. 4,406,505 (Avramovich 1983) and U.S. Pat. No. 4,961,712 (Schwenk et al., 1990).
One might consider using a washer so thick that the teeth are unable to spring back to the flat condition; however, such a piece would not easily fit in between two metal pieces and may cause the two metal pieces to deform. Also since a thick washer requires more material, it would be more expensive.
A better washer would be constructed so that the teeth are at right angles to the metal pieces to which contact should be made. Then, when the washer is tightened against the metal pieces, the teeth will not be able to spring back into the plane of the washer and will be forced to embed themselves into the adjacent piece. In embedding, both the teeth and the metal piece being contacted will deform together, resulting in a contact area where air is excluded so that the contact area will not be subject to corrosion. The height of the teeth can be designed so that the washer is guaranteed to punch through a specified thickness of anodizing. The shape and number of teeth can be designed to give a desired contact area and therefore a desired electrical resistance. The contact could be made along the inner diameter of a hole in the metal pieces; however, that might limit the amount of contact area available so it would be preferable to contact the metal pieces on an exposed face.
The washer of U.S. Pat. No. 5,828,008 (Lockwood et al., 1998) does have teeth at right angles to the piece to be contacted; however, they are only on one face. For use with machine screws, two of these washers are required. In that case, the conduction path is from the first metal piece to be connected, to the first washer, then to the machine screw, then to the second washer, then to the second metal piece to be connected. Since the conduction path has four places where current transitions between parts, there is great likelihood of a poor connection at one of the transition points and hence poor reliability. In addition, the total connection path is much longer than necessary and therefore will have relatively high resistance. A final problem is that the screw must generally be made of stainless steel to resist corrosion. The relatively poor electrical conductivity of stainless steel and the long path through the screw will limit the amount of current which can be safely conducted.
In the case of Lockwood, the washer is constructed with a concave shape. As the fastener is tightened, the washer will flatten. This change in shape will give a radial motion to the teeth which will scrape across the metal surface. For anodized surfaces the scraping may not be enough to break through the coating. It would be preferable to have the teeth directly penetrate some distance into the metal.
U.S. Pat. No. 5,435,746 (Leeb 1995) and associated U.S. Pat. No. 5,501,008 (1996) also use an electrical contact at right angles to the piece to be contacted, but it is intended for use with specialized assembly tooling and an adhesive which makes the connection permanent. In the case of application to solar modules, it is necessary to provide for module repair or replacement and a permanent connection is inappropriate.
One might consider use of the electrical grounding stud as described in U.S. Pat. No. 5,207,588 (Ladouceur et al., 1993), U.S. Pat. No. 5,441,417 (1995), and U.S. Pat. No. 5,644,830 (1997). This invention has a washer with right angle features connected to the washer and an attached stud, and also has right angle features connected to the washer and an attached rivet. The stud might be inserted through a hole in the frame of the solar module, then a nut fastened to the stud to hold it in place. The rivet could then be used to join to the mounting structure. This is clearly a cumbersome assembly process. In addition, the grounding stud will be an expensive part to fabricate and since typically four connectors are required per solar module, the total cost will be prohibitive.
Another problem with using available washers is that the washer needs to be inserted in between two metal pieces. When mounting a photovoltaic module there are four mounting holes, so one would need to carefully position four washers and then align the module to the mounting pieces. This would clearly be very difficult to do. It would be preferable to have a washer with features which retain it in position.
There are a number of available washers that have features for retaining to a bolt, such as that of U.S. Pat. No. 5,620,290 (Homfeldt et al., 1997), illustrated in FIG. 1. To use such a washer one would position four bolts through the holes in the frame of a module and place a washer on each bolt. The module would then be positioned on the mounting structure, then the module secured in place with four nuts. There are several problems here. The first is that the retaining force may not be large enough, and as the large and cumbersome module is positioned, the bolts may be knocked loose. It would be preferable to retain the washer directly to the module or mount, and then insert the bolts after the pieces have been aligned. A second problem is that the available washers are of the star type construction and do not guarantee connection as detailed previously.
There are several manufacturers who make inserts that are press fit into sheet metal parts and could thus provide the desired retention feature. For the least assembly effort, it would be desirable to construct the electrical connecting washer as an integral part of such an insert. Since the press fit process will also guarantee electrical connection, only one side of the washer would need to have the electrical connecting features.
For very high volume applications, it would be desirable to integrate the washer into the fastener and therefore eliminate assembly steps.
Accordingly, several objects and advantages of the present invention are:                1. to provide a washer which will guarantee electrical continuity between two metal pieces after they have been mechanically assembled even if the pieces are made of anodized aluminum;        2. to provide a washer with a large surface contact area for low electrical resistance;        3. to provide a washer which will be retained on one metal piece so that a second metal piece may be aligned to the first metal piece for mechanical assembly without disturbing the washer;        4. to provide a washer which requires no special tooling but makes electrical connection during the mechanical assembly process.        5. to provide a washer which can be integrated with a fastener for minimum assembly time.        
Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.